A known modular construction system makes use of galvanised malleable iron fittings used to clamp intermediate parts and ends of tubes of heavy gauge water pipe. While the fittings are robust they are not visually attractive. They are also limited to tubes of nominal outside diameter which are not available in lighter gauges. The overall weight of a structure created by way of malleable iron fittings and water pipe is substantial as against one of similar size making use of thin walled tubing. The water pipe used with the malleable fittings is not suitable for certain finishes (such as chrome plate) and is not readily available in material such as stainless steel. This is a disadvantage as use of sheet and other forms of reflecting and/or bright finish material such as chromium plate and stainless steel tube are frequently used in connection with exhibition, display, and other visually significant applications where appearance is of major importance. In addition the peripheral clearance left between the end of a pipe and a fitting into which the tube passes is frequently not uniform so showing a degree of visual incongruity which can be unattractive, if not unacceptable, in a display situation.
In what follows reference is made to a coupling intended for use with circular section tubing. However tubing of other section shape can readily be used with couplings having cups of suitable complementary cross section able to receive the other section tubing. Consequently in what follows reference is made to outside diameter in relation to round section tubes and to major transverse dimension in relation to tubes of non-circular section.